Laboratory diagnosis of respiratory virus infections has become easier and more rapid with the use of shell vials as the culture method. Immunofluorescent detection and identification of viral antigens in shell vial cultures allow a much earlier diagnosis of infection and eliminates the need (found with conventional tube cultures) for daily inspection for typical cytopathic effect and for the addition of erythrocytes to detect hemadsorbing viruses. The most optimal times to stain shell vial cultures for respiratory viruses are undetermined. At many laboratories staining at 48 hours and, if negative, again at 5 days, is believed to allow the most sensitive early detection of a respiratory virus. However, clinicians often pressure laboratories for even earlier detection so clinically-relevant decisions can be made. A study was performed to assess the value of immunofluorescent staining of shell vial cultures for respiratory viruses at 18 to 24 hours (compared with 48 hours) to allow more rapid detection and reporting of positive clinical specimens. Staining of shell vials at 18 to 24 hours allowed detection of 71 percent of the specimens positive for a respiratory virus that would ordinarily not have been detected as positive until 48 hours. This more rapid detection of the virus permitted earlier intervention by the epidemiology service and implementation of appropriate infection control measures. It also allowed earlier administration of available antiviral chemotherapy and prophylaxis. Staining shell vials at 24 hours also permitted earlier detections of Rous sarcoma virus infection that were missed by rapid antigen detection methods. Further investigation will determine whether staining shell vial cultures at 3 or 4 days is sensitive enough to replace the stain at day 5.